1. Field of the Disclosure
The present disclosure relates to methods for detecting and imaging molecules that are present in a non living sample or a living organism, and in particular, detecting and imaging molecules, or compositions of molecules, that are present in low concentrations in the living organism. Embodiments of the present disclosure employ Magnetic Resonance Spectroscopic Imaging (MRSI). Embodiments of the present disclosure do not require, i.e., are free of, the use of radioactive isotopes.
2. Description of Related Art
Clinical molecular imaging has the potential to revolutionize current diagnostic and therapeutic practice by enabling in vivo detection of molecules that are biomarkers for various diseases or biological processes of interest. For example, altered levels of glucose metabolism are known to be associated with the presence of various cancers and other disease states; indeed, it is detection of in vivo glucose metabolism that forms the basis of fludeoxyglucose F 18, also known as 2-deoxy-2-[18F]fluorodeoxyglucose (hereinafter “F18DG”) Positron Emission Tomography (hereinafter “PET”). In vivo choline detection is also under study as a method of determining tumor response to chemotherapy and other forms of treatment. Other molecules of interest for cancer diagnosis/therapeutic monitoring include creatine, citrate and N-acetyl aspartate.
Additionally, complex constructs consisting of non-biological molecules such as perfluorocarbon nanoparticles decorated with surface ligands designed to specifically bind to a desired biological site have been used as biomarkers in in vivo imaging. The challenge to molecular imaging using MRSI is that in vivo concentration of target molecules (both endogenous and exogenous) is so small that detection is very difficult or even impossible under clinically feasible conditions (which conditions include using MRI scanners with reasonable field strength and reasonable time periods for the clinical scan of the sample). As a consequence, radioactive tagging of biomarker molecules using F18 and other radionuclides, has been used as a source of detectable signal using in vivo PET.
Fluorinated glucose, which is transported into cells via glycolysis, is a case in point. Cancer cells are known to have higher glycolytic rates than healthy tissue. Once in the cell, fluorinated glucose is metabolized via hexokinase to fluorinated glucose-6-phosphate and other metabolites. The fluorinated molecules are transported out of the cell at rates much lower than the metabolites stemming from ordinary (non-fluorinated) glucose metabolic pathways. As a result, the fluorinated glucose-6-phosphate can be considered “trapped” in the cell for extended periods of time (longer than 1 hour). Hence the expectation is that cell masses showing higher than background concentrations of fluorinated glucose can be quantitatively evaluated for likelihood of being cancerous.
PET F18DG has emerged over the last 30 years as a reliable technique for identifying the presence of cancerous tissue, and more recently PET F18DG has been employed for other diagnostic purposes, including the evaluation and management of patients with suspected ischemic left ventricular systolic dysfunction, and the evaluation and management of patients with certain neurological indications (such as dementia and seizure). However, the approach has the considerable drawback of subjecting the patient to a radioactive burden, allowing this method to be used only intermittently and in circumstances where the dose related radiation risks are outweighed by the benefits of the diagnostic information yielded by the PET scan. This risk-benefit analysis must be determined (by the treating physician and patient) to favor imaging, which is usually only in the case where there is known or very high-suspicion of significant pathology such as after a positive identification for cancer has already been made. In addition, the costs and risks to staff and the environment when manufacturing, distributing and employing radioactive isotopes are high.
Because the strength of the signal emitted by the radioactive isotope in F18DG is large, very small doses of F18DG are required for PET studies. By contrast F19DG is non-radioactive and biologically identical to F18DG, but under clinically safe dose levels research has suggested its key metabolite, intracellular F19DG-6-phosphate, is available is at a very low concentrations below the threshold of detection by present day. MRSI methods and systems under clinically feasible conditions (reasonable MRI field strength and reasonable clinical scan times). As a result, while F18DG is currently useful as a diagnostic imaging agent using PET, F19DG has not been shown to be clinically useful as a diagnostic imaging agent using MRSI.
To date, translation of MRSI to clinical use has been hampered by the poor signal to noise ratio (SNR) of target molecules at low concentrations, as in the example above, and/or difficulty in obtaining spectral selectivity of target molecule(s). Though moderate increases to SNR are available through various engineering improvements (such as larger magnetic fields) none of these have the potential to enable detection of in vivo biomarkers such as those described above.